This invention is concerned with axial flow rotary valves that have both inlet port and exhaust port in the same valve. In particular it applies to rotary valves that have an outside diameter less than 85% of the cylinder bore diameter and to rotary valve engines where there is one valve per cylinder. An axial flow rotary valve is defined as one in which the axis of rotation of the valve is substantially perpendicular to the cylinder axis and the flow into and out of the valve is approximately parallel to the valve axis.
In multicylinder in-line engines using axial flow rotary valves with both inlet and exhaust ports in the same valve, there are two distinct types of axial flow rotary valves. The first type has one valve per cylinder and the second type has one valve for many cylinders.
The present invention applies to axial flow rotary valves which have one valve per cylinder. In general these multicylinder arrangements have the valve axis perpendicular to the crankshaft axis. However, this does not necessarily have to be the case. In some layouts there may be reasons for having the valve axis angled to a plane that is perpendicular to the crankshaft axis.
In the second type there is one valve for all cylinders in the bank and consequently the valve axis must be parallel to the crankshaft axis. This arrangement is basically flawed as the inlet and exhaust tract length is different for every cylinder. As a consequence the engine cannot use tuned inlet and exhaust tract lengths to optimise performance, a basic feature that is required on all modern engines.
The distinction between these two types applies only to multi cylinder in-line engines only, as on single cylinder engines there must be at least one valve per cylinder. Furthermore because there is no adjacent cylinder imposing geometric constraints on a single cylinder engine there are no constraints on the orientation of the valve axis relative to the crankshaft axis.
Axial flow rotary valve arrangements of both these types have been proposed for many years. Despite this none have been successfully commercialised. This is partly due to prior art arrangements which have poor breathing, poor combustion chamber shape, poor spark plug location and low turbulence.
The following elements are essential for a modern internal combustion engine to be competitive. Firstly, it must have adequate breathing capacity i.e. it must be able to achieve high volumetric efficiency at high speed. Secondly, it must have a combustion chamber shape and an ignition source or sources located within the combustion chamber such that the flame path to the extremities of the combustion chamber is minimised and the mass burn rate is maximised. Finally, it must be capable of generating suitable in-cylinder motion of the air fuel mixture during the intake stroke and breaking this down into small scale turbulence late in the compression stroke to maximise the speed at which the flame travels through the combustion chamber.
The arrangements required to optimise these three parameters in poppet valve engines have evolved over the last hundred years and today there is general consensus on how these parameters are optimised. The axial flow rotary valve introduces many physical constraints not found on the poppet valve, which means the solutions established for the poppet valve internal combustion engine are not readily transferable to the rotary valve.
In poppet valve technology the layout of the combustion chamber has evolved over many years to a point where there is general consensus as to what constitutes an optimum arrangement. It is universally accepted that a single spark plug mounted in the centre of the cylinder is the optimum arrangement. This arrangement is optimum as it minimises the length of the flame path to the extremities of the combustion chamber and it maximises the mass burn rate as the flame approaches the walls of the cylinder.
Engines with a single rotary valve per cylinder cannot position the spark plug in the centre of the cylinder without incurring other significant compromises in the layout of the engine. In this respect all axial flow rotary valves with one valve per cylinder have an inherent disadvantage when compared to poppet valve engines with a centrally located spark plug. In order for rotary valve engines to become commercially successful this inherent disadvantage must be addressed.
In particular, the present invention addresses a combustion chamber layout for an axial flow rotary valve where both inlet and exhaust ports are incorporated in the same valve and the valve diameter is typically less than 85% of the cylinder bore diameter.
Previous axial flow rotary valve arrangements generally fall into two categories. Firstly there are the valves that have an outside diameter equal to or larger than the cylinder bore diameter. These valves typically have a diameter 1.0 to 1.3 times the cylinder bore diameter. These arrangements do not have application in any multicylinder in-line arrangement using one valve per cylinder, as the bore spacing and hence engine length would be determined by the valve diameter and not the cylinder bore diameter. Thus any multicylinder arrangements using these valves would produce engines that are unnecessarily long and uncommercial. In general their application is limited to single cylinder engines. An example of such a proposal is shown in U.S. Pat. No. 4,404,934 (Asaka et al).
In single cylinder arrangements, the valve axis is invariably located near the centre of the cylinder. When the valve diameter is equal to or greater than the bore diameter there is no space to locate the spark plug beside the valve in a conventional location. The valve is generally located some distance from the top of the piston with the plug positioned under the valve—a typical example of such an arrangement is U.S. Pat. No. 3,948,227 (Guenther). These arrangements have very poor combustion chamber shape, and struggle to achieve a satisfactory compression ratio (particularly on engines with small cylinder swept volumes) due to the large distance between the piston and the valve.
The arrangement proposed in U.S. Pat. No. 3,948,227 (Guenther) at least has the advantage that the plug is located near the centre of the cylinder. Other arrangements such as U.S. Pat. No. 4,404,934 (Asaka et al) have the plug situated under the valve and close to the cylinder wall. The combination of the combustion chamber shape and the plug location will result in very slow combustion and poor thermal efficiency.
A multi cylinder in-line axial flow rotary valve arrangement of the second type previously described is shown in PCT publication number WO96/32569 (Ramsey), where the valve axis is parallel to the crankshaft. This arrangement has the benefit that the spark plugs are not located between the cylinders rather at the side of the cylinders. The plug placement is therefore no longer constrained by the adjacent cylinder and may be placed under the valve. PCT publication number WO96/32569 (Ramsey) states that one plug is the preferred layout but that two can be used. Despite the freedom to position the plug(s) without having to consider the adjacent cylinders the plug location is very poor, adjacent as it is to the cylinder wall and recessed from the body of the combustion chamber. The poor spark plug location in combination with the combustion chamber shape would result in very poor combustion in both single cylinder and multi cylinder engines. In the multicylinder arrangement it suffers the previously discussed problem of unequal inlet and exhaust tract lengths.
Secondly there are arrangements where the valves have a diameter smaller than the cylinder bore diameter. These arrangements have the advantage that they may be used on multicylinder in-line engines, with the valve axis perpendicular to the crankshaft axis, and the spark plugs may be inserted beside the rotary valve thus overcoming the limitations inherent in designs that require the spark plug to be located below the valve.
Typically in these arrangements the rotary valve is offset to the cylinder axis thus allowing the spark plug to be located near the cylinder centre. The valve offset can be arranged to get the spark plug sufficiently close to the centre of the cylinder to achieve acceptable combustion performance. A typical example is U.S. Pat. No. 4,852,532 (Bishop) or U.S. Pat. No. 5,526,780 (Wallis).
However there are many constructional aspects of engine design that are either compromised or complicated when offset valves are used. One such example is the location of the cylinder head bolts. These are optimally positioned from a structural, geometry and head gasket perspective at the midpoint between adjacent cylinders. This is often difficult to achieve with an offset valve.
One aspect of the present invention disclosed in this application involves the use of two spark plugs per cylinder. A two plug per cylinder arrangement is shown in U.S. Pat. No. 3,945,364 (Cook). This arrangement is however not an axial flow rotary valve as the gas flow is perpendicular to the valve axis. As with PCT publication number WO96/32569 (Ramsey), the valve axis is parallel to the crankshaft axis thus enabling the plugs to be located down the side of the engine without constraint from the adjacent cylinders. These valves arrangements overcome the problem of the valve disclosed in WO96/32569 as all cylinders may have equal length inlet and exhaust tracts. However they have been demonstrated to have problems during overlap with the inlet charge being short-circuited straight into the exhaust port and the cylinder not being adequately scavenged. This arrangement has a valve diameter approximately 30% greater than the cylinder bore diameter. As a consequence the spark plugs are located under the rotary valve adjacent the cylinder walls on opposite sides of the cylinder. While this is an improvement over a single plug located at the wall, it is still a very poor solution and one, when combined with the very poor combustion chamber shape would produce very slow burn rates and poor thermal efficiency.
Also, it is well known from extensive studies of poppet valve engines over many years that the presence of small scale turbulence in the charge gases during combustion dramatically increases the flame speed through the gases.
Turbulence is very important in all engines but particularly so in rotary valve engines, where the presence of small scale turbulence could potentially greatly increase combustion speeds and help ameliorate the effects of the inevitable non optimum spark plug locations found in rotary valve engines. There is no known prior art that addresses the issue of in-cylinder flows and the generation of small scale turbulence in rotary valve engines.
Several methods have been devised in conventional poppet valve engines to generate small scale turbulence late in the compression stroke. The three main existing methods of doing this are known as “swirl”, “tumble” and “squish”. In the case of swirl and tumble this is done by creating a bulk flow field in the cylinder during the intake stroke which decays to small scale turbulence during the compression stroke.
Tumble is defined as a flow vortex in the cylinder rotating about an axis perpendicular to the cylinder axis. In an engine designed for tumble a single major vortex is established during the inlet stroke. As the piston rises on the following compression stroke the vortex is compressed until it reaches a critical aspect ratio, where it breaks into smaller vortices. As the piston continues to rise these smaller vortices continue to break up over and over again until they become small scale turbulence.
Aspect ratio is defined as the width divided by the height of an object, except for when this is less than one, when the reciprocal (height divided by width) is used. When the piston is at bottom dead centre (bdc), the aspect ratio for oversquare engines is given by the bore divided by the stroke.
In another aspect, the present invention is concerned with methods for generating tumble in rotary valve engines. When considering tumble, two types of engine must be considered. Firstly, those with conventional bore stroke ratios of approximately 1:1 and secondly, those with high bore stroke ratios. There is no known prior art teaching on how to generate tumble in axial flow rotary valve engines with either conventional bore stroke ratio or high bore stroke ratio.
Most commercially available engines have bore stroke ratios around 1:1. In these engines the aspect ratio when the piston is at bottom dead centre is 1, which is conducive to the formation of a single major tumble vortex.
High speed engines however use oversquare engines (ie where the bore is greater than the stroke) in order to reduce the acceleration the piston and rods are subjected to at maximum engine speed. They are said to have high bore stroke ratios. For the purpose of this application an engine with a high bore stroke ratio is defined as one that has a bore stroke ratio greater than 1.4:1.
Engines using rotary valves of the type disclosed in this application potentially have very high breathing capacity, and are particularly well suited to use in high speed engines. However, the aspect ratio when the piston is at bottom dead centre is greater than 1.4 and this is not conducive to the formation of a single tumble vortex.
Squish is defined as a jet of gas acting along the piston crown shortly before top dead centre (tdc). As the piston travels towards tdc, the gas that is trapped in the areas where the piston crown and adjacent head surface come into close proximity is forced to flow at high velocity across the piston crown into regions where the piston and head are not in close proximity. Those areas where the piston crown and the adjacent head face come into close proximity to one another at tdc are known as squish zones.
In poppet valve engines squish is conventionally used as a method of generating small scale turbulence late in the compression stroke. Certain rotary valve prior art drawings shows areas of squish and the details and location of the squish suggest that it is designed to act in the same fashion that it is conventionally used on poppet valve engines. There are no known disclosures indicating the contrary.
As discussed above rotary valve engines require higher than normal levels of small scale turbulence than typically found in poppet valves. Absence of this higher level of turbulence will result in low flame speed and poor thermal efficiency. At best, the known prior art shows conventional use of squish which on its own will produce low levels of small scale turbulence.
The present invention seeks to overcome one or more of the disadvantages associated with the above mentioned prior art rotary valves.